This section provides background information related to the present disclosure which is not necessarily prior art.
A drive train with switch-on/switch-off all-wheel drive may for example comprise a power transfer unit, by means of which a drive torque can be divided on demand between a number of axles of the vehicle. For example in the case of the power transfer unit described in DE 10 2008 032 477 A1, a coupling unit which is also referred to as an all-wheel clutch is used for this, by means of which a variable proportion of a torque can be transmitted on demand from an input shaft to a secondary axle of the vehicle. In the case of a so-called “torque-on-demand” power transfer unit, the wheels of the primary axle are permanently driven, whereas a variable proportion of the drive torque can be transmitted on demand to the wheels of the secondary axle by means of the coupling unit mentioned. The torque transmission to the secondary axle takes place in this case over a torque transmission section of the drive train, which may for example be, inter alia, a cardan shaft, including an axle drive, connected thereto in terms of drive, along with a differential of the secondary axle.
Since the torque transmission section in question also rotates when the all-wheel drive is switched off, because it is dragged along by the secondary axle, to reduce undesired frictional losses when the all-wheel drive is switched off the torque transmission section may be shut down by a shutting-down device, as described for example in the documents DE 10 2009 005 378 B4 and WO 2014/166819 A2. This shutting-down functionality is occasionally also referred to by the term “disconnect”. Provided in particular in this case for achieving the disconnect functionality in question is a separating or disconnecting clutch, which in the closed state establishes a connection that is effective in terms of drive between the torque transmission section and the secondary axle, whereas the drive connection between the torque transmission section and the secondary axle is interrupted when the disconnect clutch is open. If, therefore, with the all-wheel drive switched off, or in two-wheel drive mode, the disconnect clutch is opened, this has the effect that the torque transmission section in question is uncoupled from the drive train, and in particular from the secondary axle, and rotates freely, until its rotation stops as a result of drag torques that are present. The drag torque of the torque transmission section is in this case dependent, inter alia, on the rotational speed of the torque transmission section, since for example its bearing friction tends to increase with increasing rotational speed.
If, once the all-wheel drive has been switched off and the torque transmission section has been shut down, the secondary axle is to be switched on again for purposes of all-wheel drive, on the one hand the disconnect clutch and on the other hand the all-wheel clutch must be engaged again, in order to establish a connection that is effective in terms of drive between the primary axle and the secondary axle. However, before the disconnect clutch can be engaged, the torque transmission section must be set in rotation again by means of the all-wheel clutch and synchronized with the rotation of the secondary axle. When doing so, the torque transmission section should be accelerated uniformly, and preferably with constant acceleration, in order to be able to predict as exactly as possible the time at which the disconnect clutch is to be engaged. Therefore, exact knowledge of the speed-dependent profile of the drag curve of the torque transmission section is needed in order to be able to engage the all-wheel clutch on the basis of the speed-dependent profile of the drag torque of the torque transmission section in such a way that the torque transmission section is accelerated as uniformly as possible, and in order to be able to predict as exactly as possible the time for the closing of the disconnect clutch.
The drag torque curve can in fact be determined in advance on a test bench; since, however, the drag torque curve is subject to various variable influences, such as for example varying bearing friction, other production-related tolerances, temperature influences or for example effects of wear, it would be desirable to be able to update and correct the drag torque curve of a torque transmission section of a vehicle drive train while in the process of driving the vehicle.
The invention is consequently based on the object of satisfying this desire.